That might be because the guys from fresh out of school are picking up the hot chip designer jobs but all they learned was VHDL and SPICE, no solder irons or real scopes involved. They don't make the Robert Widlars and Bob Peases anymore these days. I am seeing the same trend in circuit level design which is my kind of turf. Won't complain because when the nicely simulated "solution" goes kablouei they need consulting help :-)))
John Larkin schrieb:
Everything < 1 GHz = DC? Grmmpf!
Embarrassed ;-)
Reinhard
how much more do you need, another 10-20db ? or a lot more still ? that might be difficult ..
Im not sure what the shot noise from the detector would be, but although your signal is dc-100mhz what is the bandwitdh of the change?
I expect you could do some noise reduction on the difference signal to get the extra margin as 100mhz bandwidth means considerable noise, thats if you could demodulate it somehow otherwise youl just have to process the heck out of it with a DSP I gues.
Colin =^.^=
"colin" wrote in news:ZFZ9h.27068$ snipped-for-privacy@newsfe3-win.ntli.net:
Or you could just cool the photodiode with a TEC. Joerg said that the laser might make more noise than the detector though, so that might not help.
Photodiodes aren't very noisy; usually the amp's noise wins big-time. In Joerg's case, laser noise might be the real winner.
The light coming into the pd is photonized, so has shot noise, but improving the detector can't help that.
John
Hello Colin,
In the amp I see no problem. Never done it with a TIA chip but I designed many discrete current amps and you can hear the grass grow with those. But if the laser source is noisy, well, then it is all water under the bridge unless I can sufficiently "de-noise" that source.
It's almost the whole range :-(
The dark current noise typically wins AFAICT. The last calcs had the shot noise almost a order of magnitude above everything else. That's for the photo diode. Thing is, the RIN for the laser diode doesn't mean much for us because this app is also sensitive to phase noise. All they give you in the datasheet is spectral line width and who knows what's underneath that number. Mode jumps etc.
There will be plenty of math horsepower in that machine.
Looks like it. When they state 10MHz line width that doesn't sound encouraging. Who knows what's hiding behind that number. Guess we'll find out soon. If only they had some graphs in them datasheets.
I always wondered why nobody seems to offer the common base stages I do in discretes as a chip but that dominant shot noise may just be the reason. Maybe nobody really needs the stellar noise figures of a lone RF transistor. Although, considering that most fast TIA chips are above $2 I still think there'd be a market (my amps were always much less).
BTW, an engineer from Europe mentioned a small chip company that I almost forgot about:
They have a US office. But AFAICT no offerings for the receive side.
change?
can you compensate by monitoring the laser diode output noise ? or is the laser diode miles away ? or maybe cancel it out with feedback ?
Colin =^.^=
Yes, we could, but only in the final design. I am already seeing it coming that I have to regulate out at least some of the noise. Not looking forward to that because it has to be done up to 100MHz plus that still doesn't take care of the phase noise part. The latter would require some nifty optical detector (getting stomach cramps here...).
Good luck ! and dont forget to breath.
Would you digitise the full 100Mhz ? Wonder if the sampling units in such scopes are easily available.
Is there any structure to the signal and difference ? Ie is the full bandwidth used all the time so it just looks like white noise ? or are there periodic repetitions etc ? Is the difference in light signal due to the changes in reflection/refraction/absorbtion ?
Ive been looking for an elusive ~1ps variation in delay for some time now on and off with little success so far.
Colin =^.^=
Yes, but it won't be a scope. It's going to be our own design. Scope don't show a lot of ENOBs up there, maybe 6-7. We need a lot more.
All the time.
Basically all reflection.
Well, we'll be looking at femtometers ;-)
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